We propose to psychophysically investigate sensitivity regulation in early and intermediate-level cortical mechanisms of pattern vision. This proposal focuses on the role of spatially displaced, or contextual stimuli in the regulatory process. Spatial mechanisms in primary visual cortex have been previously characterized as sets of independent, parallel, linear filters. Physiological and psychophysical evidence now suggests that stimuli that do not directly excite a neuron can profoundly affect its sensitivity via indirect, nonlinear interactions. The proposed studies synthesize our own previous quantitative work on nonlinear sensitivity regulation in higher-level spatial mechanisms with recent psychophysical work on contextual influences on the perceptual appearance of modulated stimuli. We measure the ability to make fine discriminations between stimuli on the basis of orientation or spatial frequency, using both simple stimuli that isolate mechanisms akin to linear filters at Vi, and more complex stimuli designed to isolate higher-level pathways that are specialized to process each type of information. We measure how contextual stimuli affect sensitivity in the neurons mediating these tasks, and test whether existing models of sensitivity regulation (contrast gain control) can be extended to account for these effects. We test the hypothesis that properties of contextual stimuli (contrast, phase, spatial frequency, orientation) are critical to the interaction process at early levels of cortical processing. We test the hypothesis that sensitivity regulators operate at more than a single levels of processing, and that at higher levels, tuning is broader that at lower levels of processing. We further test the hypothesis that at higher levels of processing, contextual stimuli affecting sensitivity differs for the two tasks of spatial frequency and orientation. Finally, we test whether contextual influences depend upon figure-ground organization, and whether existing models that assume reciprocity of interactions are sufficient. These experiments will provide the empirical foundation and initial theoretical tests to advance our understanding of sensitivity regulation in pattern vision mechanisms. An understanding of these mechanisms, and the development of models of these mechanisms, is a necessary step in detecting and correcting visual deficits that disrupt pattern processing such as those found in strabismus and amblyopia.